Patent GB-1360508-A (GB)
Title: Telephone intercommunication system
IPC: H04Q3/52
Filed: 1971-10-28
Abstract: 1360508 Automatic exchange systems INTERNATIONAL STANDARD ELECTRIC CORP 28 Oct 1971 [2 Nov 1970] 50084/71 Heading H4K In an interconnection system utilizing a single stage switching network for selectively connecting calling and called lines, each line circuit includes a constant current source for feeding its line and for maintaining a connection, and, within the network, each connection is connected to ground which provides a current return path to the current source. General description.-The system includes a plurality of stations 10a to 10n which each have an individual appearance on one row of a transistor crosspoint matrix 40x4. The latter has four columns connected to four so-called link circuits 51-54 that serve to control the setting up of a speech path, to which the links are connected in shunt so as to provide a return current ground connection. The links are allocated for use by a link scanner 41 which marks one link as free pending its seizure for a new call. A continuously acting line scanner 42, on recognizing a calling line, temporarily stops so as to permit a connection for the line to be set up and then recommences its scan. Whenever a station goes off hook, a lamp lights at each station. Stations are selected by two digits which may be sent by a rotary dial or a V.F. key set and which are received in a common digit register 60. The digits, via part of the line scanner's gate circuitry, mark the called line thereby enabling it, if free, to be connected to the same link circuit to which the caller is already connected. Ringing current is supplied at the called line circuit and ringing tone or busy tone is supplied at the link. Since a wanted party is connected to a caller's link whether or not he is busy, the busy tone serves to warn him of an intruder. On reply, the link merely serves to provide a ground connection and does not form part of the speech path. When the call is set-up, only the calling and called parties lamps remain alight, these being flashed during ringing. Master stations have access to central office lines 402 and ordinary stations may reach this by courtesy of the masters via so-called special control lines 70. Line circuit (Figs. 4A, 4B) comprises an amplifier in a (non-speech) A wire for responding to an off-hook signal so as to enable a ring trip circuit i.e. to disable relay 342, when the line is called, or, in any other circumstance, to mark the line as calling by enabling gate 124 in conjunction with signals from a line scanner. Successful connection to a link is determined by a link detector 180 which, in the called state, permits ring relay 342 to come up. Current for maintaining the connection extends from constant current source 354 over the R wire via the sub set circuitry to the T wire and thence to the matrix crosspoints. CR circuit 350 prevents the ring relay 350 from reoperating at the end of a call when the handset is restored. Matrix (Fig. 5) comprises a column of crosspoints, e.g. 52-4, 22-1 individual to each subscriber, e.g. 10a, and a row of crosspoints, e.g. 52-4, 50-4 individual to each link. Each crosspoint consists of two transistors, e.g. 150, 152 that are connected to the speech wires TR L  via respective isolation diodes 154, 156 and are coupled via anticrosstalk resistors 162, 164. Bias is provided by individual resistors 158, 160. A crosspoint is actuated by a marking on the link's T, L wires in conjunction with markings on the subscriber's T 1 , R L  wires be the subscriber calling or called. In both cases 152 turns-on followed by 150 but in the former case T 1  carries a - 24 volt marking so that the crosspoint is latched until (either) the caller (or the called party if he has answered the call) hangs up whereas in the latter case T is not marked and the crosspoint will release as soon as the calling party hangs up. Operation.-The common control initially starts link allotter 41 (Fig. 2b) which thereupon marks each L1A lead in turn. In one of the links (Fig. 3) gate 102 will be open thus indicating the free state so that a signal will be returned over lead L1B to stop the allotter. After a delay to ensure the full release of the link from a preceding call, the L1A mark is effective to turn-on transistor 110 thus connecting the link's T lead to earth via choke 265 which prevents speech currents from leaking to ground. In addition lead L3 is enabled thus causing a mark to be applied to the link's L1 lead. The link is now ready to be taken into use by the next call. The lines circuits are cyclically interrogated over the tens TN and units UN leads of a line scanner 42 (Fig. 2a) so that if a calling line is encountered a gate 124 in the line circuit is opened. The scanner is therefore stopped (lead FB and scanner control 130). Furthermore the potential on the lead RL of the line's matrix appearance, in conjunction with the pre-existing markings from the link, causes the thus defined crosspoint to operate. This through-connection to the link is detected by detector 180 which thereupon busies the line gate 124. In addition a detector 201 in the link responds to the appearance of a 25 mA constant current on the through connected T wire so as to open gate 204 and thereby provide a new stop signal CLR for the line scanner. After a short delay, dialling tone is reverted over the T wire from a source in central control (Fig. 2B) via the prepared switch 94 in the link. The first incoming (tens) digit, which if it is in V.F format reaches decoder 260 via link switch 94 and common control hybrid 222 and which if it is in loop disconnect format reaches counter 304 via link current detector 201, the S lead and common control buffer 302, is decoded by relays 212 and inserted in tens store 290. Dial tone is discontinued and the decoder 212 is reset. The units digit is similarly received but is used to mark a units gate of the line scanner directly. Buffer and latch circuit 324, Fig. 2a, is thus enabled so as to read out the stored tens digit and also over lead CC to signal the link circuit that all the dialled digits have been received. Within the called line circuit, its gate 124 is now marked over the appropriate tens and units leads as well as over a start lead LS so that whether or not the line is free its RL lead is marked and therefore a crosspoint connection to the link is established. If the line is in fact busy, the potential on FB is such as to enable busy test circuit 332 in common control whereby latch circuit 90 in the link is energized. This in conjunction with the energized latch CC circuit 92, causes tone selector 320 to inject busy tone into the link's T wire. This tone will be heard by the caller as well as by the existing connection. The caller hangs up and detector 201 causes release of the link and common control circuit. It is not apparent whether the link is immediately reallotted or whether it must take its turn in the link allotter's cycle of operations. However if the called line is idle, no signal appears on TB so that tone selector 320 connects ringing tone to the link's T wire and hence to the caller's handset. In addition the link allotter is informed of the semi-successful conclusion of the operation and it therefore steps-on to find the next idle link. Ringing current for the called party is provided over lead RT in his line circuit, the contact 344 being closed by relay 342's response to the potential on wire T as detected by detector 180. When the called party replies, relay 342 is de-energized and his constant current source 354 is connected via his substation to the T wire. In the link, detector 352 responds so as to cut off ringing tone. At the end of the call, hang-up by either party is detected by the A detector 201 which thereupon turns off transistor 110 thereby releasing both crosspoints. A simplified schematic Fig. 6 depicts the system just described in respect of a single call set-up therethrough. Although not described, Fig. 3, depicts a light emitting diode LED connected in the link circuit as a busy lamp.